1. Field
The present invention relates to a cognitive radio system which uses a frequency band licensed to a particular system such as a broadcasting system, only at a time and place which are not used by the system.
2. Related Art
In order to cope with tightness of frequency resources, a study for a system called “cognitive radio” as a next-generation mobile communication system has been started. The basic concept of cognitive radio technology is that a usable frequency band is scanned to detect a use-state of a wireless channel (frequency), and a usable frequency band is borrowed. Of course, in a radio system in which cognitive radio is used (hereafter, such a system is referred to as “cognitive radio system”), a wireless channel have to be shared so as not to interfere with existing radio systems such as a broadcasting system, a radar system, and a radio astronomy observing system.
Japanese Patent No. 3,583,962 discloses a technique of sharing a frequency wireless channel for a television broadcast as a radio system.
In Japanese Patent No. 3,583,962, only a television broadcast is considered as an existing licensed radio system. In a cognitive radio system, it is expected that each wireless terminal device having a function of cognitive radio detects a wireless-channel use-state. However, there are a lot of information which cannot be obtained by a wireless terminal device alone, such as wireless-channel use-states of radio astronomy and a television receiver which perform only reception.
In the case where wireless terminal devices of cognitive radio which are separated from each other by 100 m communicate with each other, for example, other wireless terminal devices which mutually perform transmission and reception through a distance that is very shorter than 50 m, e.g., 3 m may exist at intermediate positions which are separated from the wireless terminal devices by about 50 m. There is a possibility in which radio waves from the other wireless terminal devices cannot be detected by the wireless terminal devices of cognitive radio which are separated by 50 m. When the wireless terminal devices fail to aware the radio waves and start communication with using a frequency which is used by the other wireless terminal devices, the communication obstructs the communication between the other wireless terminal devices. Therefore, a frequency by which a wireless terminal device of cognitive radio starts communication have to be determined while the terminal device knows as correctly as possible use-states of frequencies (wireless channels) in the radio-wave arrival range. However, such use-states are hardly known by a wireless terminal device alone.